Regarding a liquid crystal display device, based on the displaying mode of the liquid crystal, it can be classified into the types of PC (phase change), TN (twisted nematic), STN (super twisted nematic), ECB (electrically controlled birefringence), OCB (optically compensated bend), IPS (in-plane switching), VA (vertical alignment) and the like. Based on the driving mode of the device, it can be classified into the PM (passive matrix) type and the AM (active matrix) type. PM is classified into the static type, multiplex type and so forth, and AM is classified into TFT (thin film transistor) type, MIM (metal insulator metal) type and so forth. The types of TFT comprise amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to the manufacturing process. Based on the type of the light source, the liquid crystal display device is classified into a reflection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both the natural light and the backlight.
Liquid crystal materials are required to have good chemical and heat stability, as well as the good stabilities to electric field and electromagnetic radiation. In addition, the liquid crystal materials should have low viscosity, short response time, low threshold voltage and high contrast ratio. Various performance index of the liquid crystal composition are improved so as to obtain AM devices with good characteristics. The relationships between one performance of the liquid crystal composition and the corresponding performance of the AM device are summarized in Table 1 below. The various performance index of the composition will be further explained based on a commercially available AM device. The temperature range of a nematic phase relates to the working temperature range of the device. A desirable upper limit temperature of the nematic phase is 70° C. or more, and a desirable lower limit temperature of the nematic phase is −10° C. or less. The viscosity of the composition relates to the response time of the device. A short response time of the device is desirable for displaying a moving image in the device. Accordingly, a small viscosity of the composition is desirable. A small viscosity of the composition at a low temperature is more desirable.
TABLE 1The general characteristics of the composition and the AM deviceThe general characteristicsThe general characteristicsNo.of the compositionof the AM device1Temperature range of aUsable temperature range isnematic phase is wide.wide.2Viscosity is small.Response time is short.3Optical anisotropy is suitable.Contrast ratio is large.4Dielectric anisotropy isThreshold voltage is low,positively or negativelyelectric power consumption islarge.small, and a contrast ratio islarge.5Specific resistance is large.Voltage holding ratio is large,and a contrast ratio is large.6Stable to ultraviolet lightService life is long.and heat.
The optical anisotropy of the composition relates to the contrast ratio of the device. The product value (Δn*d) of the optical anisotropy (Δn) of the liquid crystal composition and the thickness (d) of the liquid crystal layer can be designed to be a fixed value. A suitable product value depends on the kind of operation mode. In a device having a TN mode, a suitable value is approximately 0.45 μm. In this case, a composition having a large optical anisotropy is desirable for a device having a small thickness of the liquid crystal layer. A large dielectric anisotropy of the composition contributes to a low threshold voltage, a small electric power consumption and a large contrast ratio of the device. Accordingly, a large dielectric anisotropy is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the device. Accordingly, a composition having a large specific resistance is desirable at room temperature and also at a high temperature in the initial stage. A composition having a large specific resistance at room temperature and also at a high temperature is desirable after it has been used for a long time. A stability of the composition to an ultraviolet light and heat relates to a service life of the liquid crystal display device. The service life of the device is long when the stability is high. These characteristics are desirable for an AM device used for a liquid crystal projector, a liquid crystal television and so forth.
A desirable AM device is characterized as having a usable temperature range that is wide, a response time that is short, a contrast ratio that is large, a threshold voltage that is low, a voltage holding ratio that is large, a service life that is long, and so forth. Even a one millisecond shorter response time is desirable. Thus, the composition having characteristics such as a high upper limit temperature of a nematic phase, a low lower limit temperature of a nematic phase, a small viscosity, a large optical anisotropy, a large dielectric anisotropy, a large specific resistance, a high stability to an ultraviolet light, a high stability to heat, and so forth is especially desirable.
A single liquid crystal compound is usually difficult to present its characteristics, thus normally it is mixed with a variety of other liquid crystal compounds to prepare a composition. Therefore, we need a liquid crystal composition having the above advantages. A compound mentioned in CN Publication No. CN1823151A was formulated into a liquid crystal composition and would better characteristics. The structure of the compound is as follows:

However, the dielectric anisotropy of the compound mentioned in this patent application is still not large enough.
The inventive structure in EP Publication No. EP2292720A1 is as follows:

However, the compounds mentioned in this document can not satisfy the requirements of the present invention alone.
One of the purposes of the present invention is to provide a liquid crystal composition, which satisfies at least one characteristic among the characteristics of high upper limit temperature of a nematic phase, low lower limit temperature of a nematic phase, small viscosity, large optical anisotropy, large dielectric anisotropy, large specific resistance, high stability to ultraviolet light, high stability to heat and so on. Another purpose of the present invention is a liquid crystal composition which is properly balanced regarding at least two of the characteristics. Another purpose of the present invention is a liquid crystal display device containing such composition. Another purpose of the present invention is an AM device which comprises a composition having the characteristics of large optical anisotropy, large dielectric anisotropy, high stability to ultraviolet light and so on, and the AM device has the characteristics of short response time, large voltage holding ratio, large contrast ratio, long service life and so forth.